1. Field of the Invention
The present invention relates to a method for deciding region of .pi./4 shift quadriphase shift keying (QPSK) modulated signals used for demodulation of a signal which modulated by .pi./4 shift QPSK modulating method.
2. Prior Art
FIG. 6 shows a block diagram of the structure of a usual decision apparatus of prior art for .pi./4 shift QPSK modulated signals. According to this FIG. 6, an input signal is provided by input terminal 1, an A/D converter 2 coverts the input signal to a digital signal S, a signal cos(.omega.ct) is produced by a generator 3, a .pi./2 phase shifter 4 generates a signal sin(.omega.ct) by shifting the phase of the signal cos(.omega.ct) by .pi./2.
A mixer 5 multiplies the signal S and the signal cos(.omega.ct), a mixer 6 multiplies the signal S and the signal sin(.omega.ct), low pass filters (LPF) 7 and 8 respectively limit the bandwidth of the output signals of the mixers 5 and 6, a decision circuit 9 decides on the basis of the output signals from the LPF 7 and 8 to which of the 8 parts of the phase space, indicated in FIG. 7, the input signal belongs to, the decision result coming from the decision circuit 9 is put out to a output terminal 10.
Furthermore, the decision of the decision circuit 9 is carried out as shown below. In FIG. 8, four waveforms are shown, they correspond to the four equations listed below. In FIG. 8, a curved line a corresponds to wavefunction (1), a curved line b corresponds to wavefunction (2), a curved line c corresponds to wavefunction (3) and a curved line d corresponds to wavefunction (4). EQU f.sub.1 =cos(.omega.) (1) EQU f.sub.2 =sin(.omega.) (2) EQU f.sub.3 =f.sub.1 +f.sub.2 =cos(.omega.)+sin(.omega.) (3) EQU f.sub.4 =f.sub.1 -f.sub.2 =cos(.omega.)-sin(.omega.) (4)
In FIG. 9, the signs of sign(f.sub.1), sign(f.sub.2), sign(f.sub.1 +f.sub.2), sign(f.sub.1 -f.sub.2) which correspond to functions (1) to (4) are listed while the phase angle varies from 0 to 2.times..pi..
Consequently, when the output signals from the LPF 7 and 8 as well as their computed sign combinations coincide with one of the combinations shown in FIG. 9, the decision circuit 9 decides to which of the eight areas 1 to 8 of FIG. 7 the input signal belongs to.
With the decision apparatus as described above, an explanation about the way of operation of the decision of the decision circuit 9 on the input signal is described with reference to a flow chart shown FIG. 10. First, the input signal inputted from the input terminal 1 is converted to the digital signal S in the A/D converter 2. Next, the digital signal S is multiplied by the signal cos(.omega.ct) in the mixer 5 and the multiple result outputs as a signal S.times.cos(.omega.ct) from the mixer 5. Concurrently, the digital signal S is multiplied by the signal sin(.omega.ct) in the mixer 6 and the multiple result outputs as a signal S.times.sin(.omega.ct) from the mixer 6. Then the decision circuit 9 proceeds with step SA1 (see FIG. 10) and substitutes the signal S.times.cos(.omega.ct) for a variable x and proceeds with step SA2.
In step SA2, the signal S.times.sin(.omega.ct) is substituted for the variable y and step SA3 follows next.
In step SA3, it is decided whether the variable x is not negative. If the result of this decision is "Yes", step SA4 follows next.
In step SA4, it is decided whether the variable y is not negative. If the result of this decision is "Yes", step SA5 follows next.
In step SA5, it is decided whether a difference (x-y) of the two variables x and y is not negative. If the result of this decision is "Yes", step SA6 follows next.
In step SA6, it is decided that, according to FIG. 9, the input signal falls into area 1 of FIG. 7, then an assigning phase P.sub.n =0 degree and step SA7 follows next.
In step SA7, a phase difference (.DELTA.=P.sub.n -P.sub.n-1), where P.sub.n is the phase of the existing input signal and P.sub.n-1 is the phase of the previous input signal, is obtained and is outputted from the output terminal 10 of FIG. 6 as the result of the decision, then this a whole operating cycle is finished.
If on the other hand the result of the decision of step SA5 is "No", that is, the difference of the variable x and the variable y, the difference (x-y) is less than 0 step SA8 follows next.
In step SA8, it is decided that, according to FIG. 9, the input signal falls into area 2 of FIG. 7, then the assigning phase P.sub.n =45 degrees and step SA7 follows next.
If on the other hand the result of the decision of step SA4 is "NO", that is, the variable y is less than 0, step SA9 follows next.
In step SA9, it is decided whether a sum (x+y) of the two variables x and y is not negative. If the result of this decision is "Yes", step SA10 follows next.
In step SA10, it is decided that, according to FIG. 9, the input signal falls into area 8 of FIG. 7, then the assigning phase P.sub.n =315 degrees and step SA7 follows next.
If on the other hand the result of the decision of step SA9 is "No", that is, the sum of the variables x and y, the sum (x+y) is less than 0, step SA11 follows next.
In step SA11, it is decided that, according to FIG. 9, the input signal falls into area 7 of FIG. 7, then the assigning phase P.sub.n =270 degrees and step SA7 follows next.
If on the other hand the result of the decision of step SA3 is "No", that is, the variable x is less than 0, step SA12 follows next.
In step SA12, it is decided whether the variable y is not negative. If the result of this decision is "Yes", step SA13 follows next.
In step SA13, it is decided whether the sum (x+y) of the two variables x and y is not negative. If the result of this decision is "Yes", step SA14 follows next.
In step SA14, it is decided that, according to FIG. 9, the input signal falls into area 3 of FIG. 7, then the assigning phase P.sub.n =90 degrees, step SA7 follows next.
If on the other hand the result of the decision of step SA13 is "No", that is, the sum of the variable x and variable y, the sum (x+y) is less than 0, step SA15 follows next.
In step SA15, it is decided that, according to FIG. 9, the input signal falls into area 4 of FIG. 7, then the assigning phase P.sub.n =135 degrees and step SA7 follows next.
If on the other hand the result of the decision of step SA12 is "No", that is, the variable y is less than 0, step SA16 follows next.
In step SA16, it is decided whether the difference (x-y) of the two variables x and y is not negative. If the result of this decision is "Yes", step SA17 follows next.
In step SA17, it is decided that, according to FIG. 9, the input signal falls into area 6 of FIG. 7, then the assigning phase P.sub.n =225 degrees and step SA7 follows next.
If on the other hand the result of the decision of step SA16 is "No", that is, the difference of the variable x and variable y, the difference (x-y) is less than 0, step SA18 follows next.
In step SA18, it is decided that, according to FIG. 9, the input signal falls into area 5 of FIG. 7, then the assigning phase P.sub.n =180 degrees and step SA7 follows next.
By the way, the above mentioned prior art about a decision apparatus for .pi./4 shift QPSK modulated signals had the weak point of involving a mere 45 degrees phase margin in it's decision.
Moreover, if the decision circuit 9 uses a Digital Signal Processor (DSP) and processes the above mentioned operation by software, the problem was that the DSP processing software is complicated.